ci(deploy-staging): bump Node heap to 4GB for nitro build (fix OOM)

nitro build OOM'd on the runner (exit 134, "ineffective mark-compacts
near heap limit") — Node's default ~2GB heap was exceeded as the build
grew. Runner has 7GB RAM, so raise --max-old-space-size to 4096. This
unblocks the backend deploy (IMAP crash + dm-push fixes).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/deploy-staging.yml

@@ -55,6 +55,11 @@ jobs:
 
       - name: Build backend (prisma generate + nitro build)
         working-directory: backend
+        # Runner hat 7 GB RAM, aber Node nimmt default nur ~2 GB Heap → nitro build
+        # OOM'te ("ineffective mark-compacts near heap limit", exit 134), seit der
+        # Build gewachsen ist. Heap auf 4 GB anheben (passt locker in die 7 GB).
+        env:
+          NODE_OPTIONS: "--max-old-space-size=4096"
         run: pnpm build
 
       - name: Tar artifact